Abstract : The multidrug resistance Salmonella Genomic Island 1 SGI1 is an integrative mobilizable element identified in several enterobacterial pathogens. This chromosomal island requires a conjugative IncA-C plasmid to be excised as a circular extrachromosomal form and conjugally mobilized in trans. Preliminary observations suggest stable maintenance of SGI1 in the host chromosome but paradoxically also incompatibility between SGI1 and IncA-C plasmids. Here, using a Salmonella enterica serovar Agona clonal bacterial population as model, we demonstrate that a Toxin-Antitoxin TA system encoded by SGI1 plays a critical role in its stable host maintenance when an IncA-C plasmid is concomitantly present. This system, designated sgiAT for Salmonella genomic island 1 Antitoxin and Toxin respectively, thus seems to play a stabilizing role in a situation where SGI1 is susceptible to be lost through plasmid IncA-C-mediated excision. Moreover and for the first time, the incompatibility between SGI1 and IncA-C plasmids was experimentally confirmed.